How much does it cost to develop a website like Yelp or Groupon?

You’re probably asking the wrong question. The initial build phase of a site like Yelp (the bit before launch) is just the start of the process—once the site has launched, it will certainly need to evolve based on how the site’s users are using it. You can’t just pay a bunch of developers to build it and then launch it without expecting to keep working on it.

So you need at least one developer (a team of 2 or 3 would be more likely), full time, for 3-6 months to get to launch—and you’ll need to keep the team on after that. You should be able to figure out the costs from there.